Conventionally, Ni, Cu and Ag brazing filler metals have been used for brazing stainless steel. Of these brazing filler metals, Cu and Ag brazing filler metals may not be adequate in some applications where the mechanical strength and the corrosion resistance of the brazed joint are required to be substantially as good as the base material because these brazing filler metals have limited mechanical strength and corrosion resistance. Ni brazing filler metal is relatively preferable for brazing stainless steel as it is favorable in both mechanical strength and corrosion resistance. Ni brazing filler metal is available as Ni--Si, Ni--B and Ni--P base brazing filler metals. Table 1 summarizes the composition, the melting point and the brazing temperature of common Ni-base brazing filler metals.
TABLE 1 __________________________________________________________________________ solid liquid phase phase brazing composition (%) line line temp. Ni Cr B Si Fe P (.degree. C.) (.degree. C.) (.degree. C.) __________________________________________________________________________ BNi-2 bal. 13.0-15.0 2.75-3.5 4.0-5.0 2.0-4.0 970 1,000 1,010-1,175 (Ni--B) BNi-5 bal. 18.0-20.0 -- 9.75-10.5 1,080 1,135 1,150-1,205 (Ni--Si) BNi-6 bal. -- -- 10.0-12.0 875 875 925-1,025 (Ni--P) BNi-7 bal. 11.0-15.0 -- 9.0-11.0 890 890 925-1,040 (Ni--P) __________________________________________________________________________
Ni--Si (Bni-5) brazing filler metal contains a large quantity of Si (approximately 10% in the case of BNi-5). A small quantity of Si lowers the melting point of the brazing filler metal, but a large quantity of Si could form Ni--Si compounds in the brazed joint. As the Ni--Si compounds are relatively brittle, inclusion of such Ni--Si compounds could compromise the toughness of the brazed joint. In particular, if there is wide clearance in the brazed joint, a phase of Ni--Si compounds tends to grow to such an extent that the possibility of crack formation in this part increases. Also, because the melting point of the brazing filler metal becomes relatively high, exceeding 1,100 .degree. C., the heat for the brazing could adversely affect the mechanical properties of the brazed stainless steel.
Ni--B (BNi-2) brazing filler metal contains B so that relatively brittle Ni--B compounds could be formed in the brazed joint. Similarly as Ni--Si (BNi-5) brazing filler metal, Ni--B compounds could substantially reduce the toughness of the brazed joint. The Ni--B compounds which may separate from the material could accelerate the corrosion of the brazed joint. Ni--B brazing filler metal has a relatively low melting point, and can be brazed at a lower temperature than Ni--Si brazing filler metal. Therefore, the likelihood of adversely affecting the mechanical properties of the base material may be reduced, but the possibility cannot be totally eliminated depending on the composition of the stainless steel.
Ni--P brazing filler metal has a lowest melting point of all Ni-base brazing filler metals, but the P content makes the brazing filler metal unsuitable for use on stainless steel because the P content has a tendency to make iron materials brittle.